I noticed a rather weird behavior with transactional update, which is likely a bug.
Steps to reproduce:
1. Enter transactional-update shell (sudo transactional-update shell)
2. Create a directory inside of that shell (mkdir /exampledir)
3. Exit the shell and reboot
4. Make the directory we created above a btrfs subvolume (sudo mksubvolume /exampledir)
At this point, /exampledir is added as a mount point in /etc/fstab
5. Reboot and verify that the mount point works properly. /etc/fstab remains the same.
6. Open transactional-update shell again (sudo transactional-update shell)
At this point, you can see that /etc/fstab inside of the shell no longer has /exampledir as a mount point.
7. Exit the shell and reboot
After the reboot, you can see that /etc/fstab is changed back to how it was prior to step 4.
Even stranger, at step 6, you can manually add the mount point for /exampledir in /etc/fstab and reboot, and it should work just fine. However, should you enter the transactional-update shell again, /etc/fstab will once again be changed back to prior to step 4 and the mount point will not appear after a reboot unless you manually add it back to /etc/fstab within the shell.
I hope someone could have a look at this.